1. Field of the Invention
The present invention relates to a method and an apparatus for correctly measuring optically the thickness of a transparent thin film formed on a substrate.
2. Background of the Prior Art
In a film thickness inspection of a semiconductor manufacturing process or the like, it is necessary to measure the thickness of a silicon oxide film which is formed on a silicon substrate, for example, in a non-contact manner. In order to measure such film thickness, generally employed are a method (hereinafter referred to as the "polarization analyzing system)" of measuring a change in a polarized state of reflected light and obtaining the film thickness from such change and another method (hereinafter referred to as the "reflected light energy measuring system") of irradiating a sample to be measured with light for measuring the energy of light reflected by the sample to thereby obtain the film thickness from the energy.
In the polarization analyzing system, light is applied to a sample obliquely from above for measuring the thickness of a transparent thin film formed on the sample. Thereafter, detected are change in a polarized state of light reflected by the sample to be measured (i.e., change in relative phase shift between the P wave, which is a vibration component parallel to the plane of incidence, and the S wave, which is a vibration component perpendicular to the plane of incidence) and change in the amplitude ratio between the waves. Further, the thickness of the transparent thin film is obtained on the basis of the detected change. This polarization analyzing system is applied to an ellipsometer, for example, so that the thickness of a thin film formed on a sample to be measured can be accurately measured by the ellipsometer in a non-contact manner even if the film thickness is not more than 10 nm. However, it is difficult to measure a region to be measured while limiting the same in a narrow range. Thus, the reflected light energy measuring system, which is described below, is generally employed in a semiconductor manufacturing process.
When light is applied to a sample to be measured, light reflected by the surface of a transparent thin film formed on the sample interferes with that reflected by the surface of a substrate of the sample, whereby constant relation holds between the energy of the reflected light and the thickness of the transparent thin film. In reflected light energy measuring system, the thickness of the thin film is obtained on the basis of such relation.
In more concrete terms, the thickness of the transparent thin film is obtained as follows: First, samples (hereinafter referred to as "first standard samples") provided with transparent thin films having different thickness values are prepared in order to obtain correlation data of energy of reflected light and thickness values of the transparent thin films. Optical constants of the transparent thin films and substrates of the first standard samples are identical to those of a sample to be measured, and the thickness values of the transparent thin films of the first standard samples are known, respectively. Visible light is applied to the respective standard samples to measure energy of light reflected by the surface of the standard samples, to thereby obtain correlation data of the energy of the reflected light and the thickness values of the transparent thin films. Thus, prepreparation is completed. Then, visible light is applied to the sample to be measured, in order to measure energy of light reflected by the sample to be measured. Finally, the thickness of a thin film formed on this sample is obtained on the basis of the measured values and the correlation data.
As understood from the above, the reflected light energy must be significantly changed in response to change in film thickness, so that the film thickness can be correctly measured by the reflected light energy measuring system. In general, most of thin films formed on semiconductor substrates are at least 40 nm in thickness, and the reflected light energy is significantly changed even if the amount .DELTA.d of change in film thickness is very small. Thus, it has been possible to measure the thickness of such a thin film to a relatively high accuracy by the reflected light energy measuring system.
The thickness of a thin film has been reduced with recent improvement in semiconductor manufacturing technique, and it has been increasingly demanded to accurately measure such film thickness also within a range of not more than 10 nm. However, it is impossible to correctly measure film thickness of not more than 10 nm by the reflected light energy measuring system. The reason for this is that energy of light reflected by a thin film of not more than 10 nm in thickness is approximate to energy E.sub.O of light reflected by a sample (hereinafter referred to as "second standard sample") which is provided with no transparent thin film. In other words, energy of reflected light is substantially constant within a range of 0 to 10 nm in thickness. In more concrete terms, the energy of reflected light is theoretically provided as E in the case of film thickness of 10 nm, for example, as shown in FIG. 1, while actually measured energy of the reflected light includes a measurement error, to thereby be not identical to the theoretical value E. As understood from FIG. 1, variation in energy with respect to change in film thickness is small around film thickness of 10 nm, particularly that of not more than 10 nm. Therefore, film thickness obtained on the basis of the energy value including the aforementioned measurement error may significantly differ from the true film thickness.